environment: move editor_program into repo_config_values

The global variable 'editor_program' holds the path to the user's
preferred editor. Move 'editor_program' into
'struct repo_config_values' to continue the libification effort.

There have been discussions on whether external programs like
editors truly need to be configured on a per-repository basis within
the same process. While a single process might rarely invoke
different editors, this migration is necessary for two reasons:

1. Developers frequently use different toolchains for different
   projects. Per-repo configuration respects this.

2. Moving this string into 'repo_config_values' eliminates mutable
   global state. As the codebase moves toward becoming a long-running
   processes, managing multiple repositories concurrently must
   not overwrite each other's program configurations.

No standalone getter function is introduced. Callers directly access
the field via 'repo_config_values()'. Heap memory is safely reclaimed
in 'repo_config_values_clear()'.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Mentored-by: Olamide Caleb Bello <belkid98@gmail.com>
Signed-off-by: Tian Yuchen <cat@malon.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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